The present invention relates to a circuit for suppressing the noise produced by the switching of two voltage sources having a direct-current offset, particularly for audio preamplification stages.
A typical application of the present circuit is he autoreverse amplification stage for preamplifying signals from the magnetic head of tape recorders. An example of such an audio stage is shown in FIG. 4, in which the preamplification stage comprises an equalizing network for amplifying solely within the 20 Hz to 20 KHz audio frequency range.
In FIG. 4, the two sources are represented by three sources: source 1 for producing a first alternating-voltage signal v.sub.in1 having no direct current offset; source 2 for producing a second alternating-voltage signal v.sub.in2 having no direct current offset; and source 3, series connected to source 2, for producing the direct current offset voltage V.sub.off between the two sources. Sources 1 on the one hand and 2 and 3 on the other are connected between ground and a common point 13 by respective switches 4 and 5, which, during operation of the stage, are alternately closed. Point 13 is in turn connected to the non-inverting input of an operational amplifier 6 defining the preamplification stage together with equalizing network 7. Network 7 comprises a first line consisting of a resistor 8; a second line consisting of the series connection of resistor 9 and condenser 10, both lines being connected parallel between the output and inverting input of amplifier 6; and a third line 14 consisting of the series connection of resistor 11 and condenser 12, and connected between the same inverting input and ground.
When dc operated, the two condensers function as an open circuit, so that the stage gain is unitary. When, on the other hand, the frequency of the input signal (v.sub.in1 or v.sub.in2, depending on which of switches 4 and 5 is closed) exceeds the lower cutoff frequency 1/(2.pi.*R.sub.1 *C.sub.1), where R.sub.1 is the resistance of resistor 11 and C.sub.1 the capacitance of condenser 12, the signal at the non-inverting input of amplifier 6 is amplified.
The values of components 11 and 12 are normally selected so that the lower cutoff frequency is slightly less than the lowest audio frequency.
In a stage of the aforementioned type, switching of the input voltages produces a spurious transient state at the output of amplifier 6, which, in the case of audio applications, is unacceptable.
One known solution consists in dc decoupling of the two signal sources from the multiplexed input of amplifier 6. For this purpose, as shown in FIG. 5, provision is made, between sources 1 and 2 and respective switches 4 and 5, for a first and second high-pass filter, each consisting of a decoupling condenser 15, 16 and a decoupling resistor 17, 18. In this case, the time constants of the two filters 15, 17 and 16, 18 must be high enough to also permit the passage of the lowest audio signal frequency (in this case 20 Hz). This can be achieved in two ways: using a high-capacitance condenser 15, 16 and a resistor 17, 18 of not too high a value, or vice versa.
The first solution, though technically good in that, during normal operation, resistor 17, 18 produces very little noise at the output of amplifier 6, is expensive by requiring the use of non-integratable electrolytic condensers.
The second, though more economical, results in a high noise level at the output.
An alternative noise suppression solution consists in operating downstream from the amplifier, by providing a switch at the output of amplifier 6, controlled in such a manner as to prevent the passage of the signal during the transient state produced by switching of the voltages, but which lets the signal through during normal operation. This is shown in FIG. 6 in which the output of amplifier 6 is connected by a switch 20 to a high-pass filter comprising a decoupling condenser 21 and a decoupling resistor 22. Downstream from filter 21, 22, provision is also made for a buffer stage 23. One terminal of resistor 22 is biazed to a reference voltage V.sub.REF. Thus, stage output V.sub.O is biazed to the reference voltage during the transient state, while buffer 23 receives and supplies the amplified audio signal during normal operation.
The above solution, however, also presents a number of drawbacks in terms of complex design and cost, due to the need, once more, for a decoupling condenser; the presence of two additional pins (squares 24 in FIG. 6); and the use of an output buffer stage. A further drawback is posed by the necessity of referring the two sources (1-3), which are external components, to reference voltage V.sub.REF. If this were not so, i.e. if the input signals were referred to ground, condenser 21 would have to be charged to reference voltage V.sub.REF, which would result in a further transient state when switch 20 is closed.